consider going to HD to get some of the Styro insulating panels with reflecting foil on each side. They are fairly cheap... they come in 8x4x2(or so). Those some 2x3's and you can make a small insulated area. Then just the heat of a 1kw will do the trick. You can go from seedlings to veg on HPS and no worries about keeping the whole damn area warm. for under $100 you're all set.